Bottom Line:
Here we demonstrate that mechanical bistability caused by rapid and anisotropic adsorption and desorption of water vapour by a flexible dynamic element that harnesses the chemical potential across very small humidity gradients for perpetual motion can be effectively modulated with light.A mechanically robust material capable of rapid exchange of water with the surroundings is prepared that undergoes swift locomotion in effect to periodic shape reconfiguration with turnover frequency of <150 min(-1).The element can lift objects ∼85 times heavier and can transport cargos ∼20 times heavier than itself.

ABSTRACTHygroinduced motion is a fundamental process of energy conversion that is essential for applications that require contactless actuation in response to the day-night rhythm of atmospheric humidity. Here we demonstrate that mechanical bistability caused by rapid and anisotropic adsorption and desorption of water vapour by a flexible dynamic element that harnesses the chemical potential across very small humidity gradients for perpetual motion can be effectively modulated with light. A mechanically robust material capable of rapid exchange of water with the surroundings is prepared that undergoes swift locomotion in effect to periodic shape reconfiguration with turnover frequency of <150 min(-1). The element can lift objects ∼85 times heavier and can transport cargos ∼20 times heavier than itself. Having an azobenzene-containing conjugate as a photoactive dopant, this entirely humidity-driven self-actuation can be controlled remotely with ultraviolet light, thus setting a platform for next-generation smart biomimetic hybrids.

Mentions:
As shown in Supplementary Movie 9, when a PCAD@AG film in motion was exposed to ultraviolet light with power of 20 mW cm−2, its motion ceased instantaneously (Fig. 4a−c; Supplementary Note 3 and Supplementary Fig. 11). The motility resumed as soon as the excitation was terminated. This switching on and off of the humidity-driven motility can be repeated many times, and is brought about by the isomerization of the azobenzene chromophores in PCAD (we confirmed that a film of pure AG moved very slowly over a moist surface and ultraviolet light did not have any effect on the motion; Supplementary Movie 10). This exercise indicates the possibility to remotely ‘gate' the humidity-driven motility with light. To demonstrate the macroscopic effects of photogating of humidity-driven actuation, a bending film was exposed to ultraviolet light (Fig. 4d–g; Supplementary Movie 9). Humid air was continually supplied laterally to the film to maintain relative humidity within the range of mechanical bistability. This induced slow deflection from the source of humidity due to expansion of the contact surface (stage I, Fig. 4d; for details of the kinematic analysis see Supplementary Methods). Exposure to ultraviolet light from the opposite direction resulted in expansion of the illuminated surface and suppression of the humidity-induced unidirectional bending. This action reverted the bending direction, causing the film to deflect away from the light source and against the air flow, that is, in the direction of increasing humidity gradient (stage II). After the ultraviolet light was switched off, the film switched the bending direction again and resumed its original unidirectional bending (stage III).

Mentions:
As shown in Supplementary Movie 9, when a PCAD@AG film in motion was exposed to ultraviolet light with power of 20 mW cm−2, its motion ceased instantaneously (Fig. 4a−c; Supplementary Note 3 and Supplementary Fig. 11). The motility resumed as soon as the excitation was terminated. This switching on and off of the humidity-driven motility can be repeated many times, and is brought about by the isomerization of the azobenzene chromophores in PCAD (we confirmed that a film of pure AG moved very slowly over a moist surface and ultraviolet light did not have any effect on the motion; Supplementary Movie 10). This exercise indicates the possibility to remotely ‘gate' the humidity-driven motility with light. To demonstrate the macroscopic effects of photogating of humidity-driven actuation, a bending film was exposed to ultraviolet light (Fig. 4d–g; Supplementary Movie 9). Humid air was continually supplied laterally to the film to maintain relative humidity within the range of mechanical bistability. This induced slow deflection from the source of humidity due to expansion of the contact surface (stage I, Fig. 4d; for details of the kinematic analysis see Supplementary Methods). Exposure to ultraviolet light from the opposite direction resulted in expansion of the illuminated surface and suppression of the humidity-induced unidirectional bending. This action reverted the bending direction, causing the film to deflect away from the light source and against the air flow, that is, in the direction of increasing humidity gradient (stage II). After the ultraviolet light was switched off, the film switched the bending direction again and resumed its original unidirectional bending (stage III).

Bottom Line:
Here we demonstrate that mechanical bistability caused by rapid and anisotropic adsorption and desorption of water vapour by a flexible dynamic element that harnesses the chemical potential across very small humidity gradients for perpetual motion can be effectively modulated with light.A mechanically robust material capable of rapid exchange of water with the surroundings is prepared that undergoes swift locomotion in effect to periodic shape reconfiguration with turnover frequency of <150 min(-1).The element can lift objects ∼85 times heavier and can transport cargos ∼20 times heavier than itself.

ABSTRACTHygroinduced motion is a fundamental process of energy conversion that is essential for applications that require contactless actuation in response to the day-night rhythm of atmospheric humidity. Here we demonstrate that mechanical bistability caused by rapid and anisotropic adsorption and desorption of water vapour by a flexible dynamic element that harnesses the chemical potential across very small humidity gradients for perpetual motion can be effectively modulated with light. A mechanically robust material capable of rapid exchange of water with the surroundings is prepared that undergoes swift locomotion in effect to periodic shape reconfiguration with turnover frequency of <150 min(-1). The element can lift objects ∼85 times heavier and can transport cargos ∼20 times heavier than itself. Having an azobenzene-containing conjugate as a photoactive dopant, this entirely humidity-driven self-actuation can be controlled remotely with ultraviolet light, thus setting a platform for next-generation smart biomimetic hybrids.